1. Field of the Invention
The present invention relates to an optical recording medium having at least one light transmissive layer having an information recording layer and formed on one main surface of a substrate and used for reproducing information by irradiation of reproduction light from a light transmissive layer side.
More particularly, the present invention relates to an optical recording medium which can have a larger capacity by determining a relationship among a thickness of the light transmissive layer, a thickness unevenness, a skew and so on, and an optical disk apparatus for recording or recording and reproducing the optical recording medium.
2. Background of the Invention
An optical recording medium on one side of which an NTSC signal data of four hours amount can be recorded and reproduced has been proposed as that for the next generation. Use of this proposed optical medium allows a home-use video disk recorder to record and reproduce data for four hours, thereby the above optical medium having a function of a new recording medium which can be replaced with a cassette used in a current video cassette recorder (VCR).
Since the above optical recording medium has the same shape and size a compact disc (CD), the above optical recording medium can be a product which prevents even a user who are accustomed to easy handling and operability of the CD from feeling storage.
Moreover, if a fast access which is a most important feature of the above optical recording medium is utilized, it is possible to realize a product which not only serves as a small-sized and easily operable recorder but also has various functions such as video recording, playback, a trick play, edition and so on that can be carried out instantaneously.
Realization of such product requires a storage capacity of 8 GB or larger.
However, there has been no optical recording medium which has a single-layer information recording layer only on one side and can have a storage capacity of 8 GB or larger.
A previously proposed digital versatile disc (DVD) has a storage capacity of 4.7 GB in an area of an information recording portion, i.e., an area ranging from a portion away from a disk center by 24 (mm) to a portion away therefrom by 58 (mm) in a disk radius direction when a wavelength .lambda. is 0.65 .mu.m and a numerical aperture (N.A.) is 0.6.
If a larger storage capacity is desired without any change of a signal format such as an error correction code (ECC), a modulation system and the like, establishment of the following equation (1) is required for realization of a storage capacity of 8 GB or larger. EQU 4.7.times.(0.65/0.60.times.N.A./.lambda.).sup.2 .gtoreq.8 (1)
According to the above equation (1), N.A./.lambda..gtoreq.1.20 must be established.
Study of this requirement reveals that it is necessary to set the wavelength .lambda. shorter or to set N.A. larger.
In order to satisfy the above condition, if the value of N.A. is set larger, it is necessary to decrease a thickness of a light transmissive layer of the optical recording medium through which irradiated reproduction light is transmitted.
The reason for this setting is that an allowance of an angle (tilt angle) at which a disk surface is displaced from a surface perpendicular to an optical axis of an optical pickup becomes smaller. Specifically, the tilt angle is easily affected by optical aberration resulting from a thickness of a base of the optical recording medium.
From similar reason, unevenness of a thickness of the light transmissive layer must be suppressed to a certain value or smaller.